The present invention relates to a level measuring arrangement for measuring the liquid level in a tank by using a microwave transmitting/receiving devices and a waveguide that extends into the tank for guiding a coupled-in microwave.
In addition to mechanical measuring methods using floaters, arrangements for measuring the liquid level in tanks are known and are used primarily in large tank plants and tank plants with floating lids. These arrangements use microwave radiation to determine the distance to the liquid, and thus the liquid level, from the echoes reflected by the liquid surface. The measuring accuracy is in the millimeter range to centimeter range.
An arrangement of the above type is shown in U.S. Pat. No. 5,136,299, in which a microwave transmitter excites a wave in a first hollow conductor segment. A specific polarization direction of the electric field vector is forcibly imposed on this wave by means of a polarizing plate. The polarized wave travels from the first hollow conductor segment via a transition piece into a measuring tube having a considerably larger diameter. The measuring tube has a plurality of openings along a casing line to allow an interchange with the surrounding liquid, so that it is ensured that the filling level inside the measuring tube is equal to the surrounding filling level. The tube also functions as a settling tube (stilling well) to balance rapid fluctuations of the liquid surface.
The measuring tube functions as a hollow conductor for the coupled-in microwave. For specific types of oscillation, the large tube cross section can provide propagation conditions with particularly low losses. However, it also allows for the propagation of various modes with potentially different propagation speeds. Depending on the excited type of oscillation, the openings serving as interruptions in the tube wall can, moreover, influence the propagation conditions, so that the signal analysis in the microwave device cannot be scaled linearly to the filling level despite the calibration markings that are provided. In particular, the propagation conditions in the measuring operation and the calibration operation can differ with the known arrangement.